Which it's better ?

Do you guys think it's better do linkbuild on:

a) Several websites of general content with PR 4-7

b) Several website of content related to my site with PR 1-3

I don't understand what you mean by general content and content related to my site.

You should have keyword rich content. Highest PR will do good in terms of SEO.

c) Both.

Ultimately it depends on the individual sites and pages you're getting the links from. If the niche sites are ranking in the serps then those will be some powerful, relevant links. If they're just some shitty sites with no traffic then those links will be no better than anything else.

If the general sites are high PR but the particular pages you're getting links from aren't, then those aren't going to be very good links. But high PR hardlinks from off topic pages can carry a lot of weight.

So really, just get all the links you can from every source available.

When it comes to link building, relevance of the page with your page is the first thing to focus upon.

I would stress on second option.

Let's say I have a website about dirty bikes.

By general content I mean, building links on music,travel,tech, or any other kind of website that does not relate to my theme.

By content related I mean, websites about bikes, motorcycles,adventure, classified about dirty bikes, etc...